Presidents rarely have substantial coattails during their re-election efforts, but often have them in their first election. So they tend to enter their first midterms with a large number of members in vulnerable seats. If they lose seats in the first midterm, then they’re pretty well cleared out for the second midterm. If they manage to avoid losing seats the first time, they tend to still be overexposed in their second midterm.

Similarly, it is unusual to make it through an entire eight-year term without hitting a rough patch in the economy. If a downturn happens early in the first term, things will be rough during that midterm election. But given the length of the business cycle, another downturn is unlikely to happen again in the second term (if there is one). At the same time, if a rough patch doesn’t appear in the first term at all, the odds are pretty good it will occur at some point in the second (again, given the business cycle).

This makes excellent sense when considering the House of Representatives, where Republicans picked up 60 seats in 2010. Many of those were districts originally lost in 2006 and 2008, when moderate Democrats picked up seats in the mountain west, midwest, and south.

But there is still the Senate, where seats up in 2014 have yet to experience the Tea Party surge. After the debacles of candidates like O'Donnell, Akin, and Mourdock, I'm skeptical the GOP will pick up all the seats it should, but things still look good there going into the election. (And I'm always optimistic about Republican prospects when Obama is not on the ballot.)